Fix a couple of things causing bot redness:

tools/migration/lib/src/fork.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 
 import 'io.dart';
 import 'log.dart';
@@ skipping 95 matching lines @@
 
   String _strongSource;
 
   Fork(this.twoPath);
 
   List<String> migrate() {
     print("- ${bold(twoPath)}:");
 
     var todos = <String>[];
 
-    if (!oneExists && !twoExists) {
+    if (!oneExists && !strongExists) {
       // It's already been migrated, so there's nothing to move.
       note("Is already migrated.");
     } else if (twoExists) {
       // If there is a migrated version and it's the same as an unmigrated one,
       // delete the unmigrated one.
       if (oneExists) {
         if (oneSource == twoSource) {
           deleteFile(onePath);
           done("Deleted already-migrated $onePath.");
         } else {
@@ skipping 38 matching lines @@
     }
 
     // See what work is left to be done in the migrated file.
     if (twoExists) {
       validateFile(twoPath, twoSource, todos);
     }
 
     return todos;
   }
 }